Long-term changes in phytoplankton biomass and structure were studied in three heated lakes that were included into a power plant cooling system in 1958 and 1970. Since the mid 1990s, the share of Vallisneria spiralis L., which is a thermophilic hydrophyte species that is alien to Poland, has been increasing. The phytoplankton biomass fluctuated in these basins in the 1992-2003 period from 0.2 to 49.3 mg dm -3 . Two phytoplankton assemblages were noted: one was typical of the cold season and the other of the warm season. The phytoplankton in the summer comprised cyanophytes, chlorophytes, dinoflagellates, and cryptophytes. Diatoms developed on a massive scale during cold periods. The results obtained indicate that the contemporary phytoplankton assemblages are shaped by the varied impact of water heating, increased flow rates, the highly productive waters, and the developing submerged hydrophyte phytocenosis along with the abundant growth of the epiphyte assemblage.